The invention relates to a claw-pole stepper motor having a stator which comprises at least two stator return elements bounded by outer sides, which elements each accommodate at least one stator coil, and having a rotor which comprises a permanent magnet and which is axially supported by means of at least one axial bearing, the rotor having an inner screwthread in which a threaded spindle is engageable.
Such a claw-pole stepper motor is known from EP 550 102 A2. Claw-pole stepper motors have no winding overhangs and the stator is bounded by a stator return element, for example a stator plate, at both axial ends. As a result, the axial length of the permanent magnet of the rotor of such a known claw-pole stepper motor is smaller than or equal to the axial distance between the outer ends of the stator return elements, in order to minimize the amount of permanent-magnet material. If the threaded spindle is locked against rotation a rotation of the rotor will be converted into an axial movement of the threaded spindle. If the threaded spindle is used, for example, to drive an actuating member, oppositely directed reaction forces will act upon the threaded spindle during its axial movement. These axial reaction forces may cause a displacement of the rotor in an axial direction owing to bearing tolerances of the axial bearing. As a result of such an axial displacement of the rotor, the electromagnetic flux generated by one of the two stator coils is no longer completely linked to the rotor, which leads to a reduction of the torque produced by the motor. In order to preclude the axial displacement of the rotor bearings having a very high axial stiffness have to be used and the claw-pole stepper motor should be manufactured with a very high mechanical precision. This results in high production costs for the motor.